X-rays are used in a number of different applications, such as medical diagnosis and treatment, inspection of parts to find hidden defects, screening of baggage and other items at sensitive areas (such as airports), and studying of very small particles.
An X-ray source typically includes an X-ray tube in which electrons are emitted from a thermionic cathode. The emitted electrons are accelerated by a large potential difference so that the electrons impact an anode. The electrons bombard the anode with sufficient energy to displace inner, more tightly bonded electrons from atoms in the anode. When these excited atoms return to their ground state, they emit short wavelength electromagnetic radiation that is known as X-rays. Conventional X-ray sources that include X-ray tubes tend to be relatively large in size, which may constrain the manner in which such X-ray sources can be used.
Particle accelerators, such as linear accelerators, are sometimes used as X-ray sources for generating relatively high-energy X-rays. However, such particle accelerators tend to be relatively expensive, and thus are not widely used.